1. Field of the Invention
The present invention relates to the extraction of tritium and preparation of radioactive waste for disposal. More particularly, the present invention relates to tritium recovery during the process of heating and melting irradiated nuclear targets.
2. Discussion of Background
Tritium is a radioactive isotope of hydrogen. Because it is frequently found in the form of a diatomic gas or bound in a water molecule, it is relatively mobile and difficult to recover and contain once it has escaped into the environment. Preventing the escape of tritium is an important priority in the operation of facilities where tritium is created as a product or a by-product.
Tritium is created when lithium metal in target bundles is irradiated in a nuclear reactor. Nuclear reactor targets, after use, are referred to as "spent". Spent target bundles must be stored in carefully constructed disposal cells for a few hundred to thousands of years depending on the length of time the radioisotopes remain significantly radioactive and on government requirements. The volume required for long-term storage of radioactive waste should be minimized because of cost and other considerations.
Using heat to drive tritium from nuclear targets for recovery is well known. See for example both U.S. Pat. No. 4,532,102 and 3,100,184 issued to Cawley and Abrahams, respectively.
A prior art furnace, crucible and target bundle is illustrated in FIG. 1. A target bundle after heating is illustrated in its crucible in FIG. 2. A target bundle 20 is placed in a crucible 22 that holds it and encloses the bundle laterally, the top of the crucible is covered with a perforated plug 24 and has an integral hook 26. Crucible 22 is lowered into a retort 34 by a lid 36 that seals against a top flange 38 of retort 34. Retort 34 is positioned in a furnace 50 having electric resistance heaters 52 inside its wall 54. An inlet 56 and an exit 58 allow the circulation of coolant to prevent the shell 60 of the furnace from overheating. An off-gas pipe 62 allows tritium to be drawn from the interior of retort 34.
After heating, bundle 20 occupies a much smaller volume than before heating, as depicted in FIG. 2 where a melted bundle 70 is shown in a crucible 72. The melted bundle, slumped to the bottom of the crucible, is disposed of still in the crucible. Although gases from the furnace are drawn off during heating, opening the furnace causes some tritium to escape to the environment.
The current furnace, therefore, is unsatisfactory because it allows too much tritium to escape. Furthermore, the melted bundle comprises only about one tenth of the volume of the crucible. Thus, a substantial fraction of the interior of the crucible, as sent to a waste repository, is empty space.